In an effort to reduce this overall moisture under my basement floor and walls, I am installing an interior weeping tile. My foundation walls have no footing (this is known from foundation repair and exterior excavation last year). The bottom of the wall is at the bottom of the floor slab level. The contractor will dig a trench 1 foot wide and 1 foot deep. The question is: How far from the wall should this trench be (again noting that there is no footing)? My understanding is that the rule is 1:3, i.e. 3 feet away for 1 foot down so as not to undermine the foundation walls. Of course, the trench will contain a sock-covered weeping tile will appropriate gravel all around.

Additionally, I would like to run some "tentacle" trenches, i.e. pieces of weeping tile perpendicular to the wall in order to get closer to the wall than the "main loop" described above. These would run from a T-connector on the main loop and then perpendicular towards the wall. If we want to put these tentacles every 5 feet, then how close can I go to the wall? An engineer indicated approximately 1 foot away from the wall for these 1 foot deep tentalces, although I think this was just a guess. Can I go closer, or even right under the wall itself?

This would be 1 foot dug for every 4 feet existing earth, etc.

Note that my soil is about 80-90% sand content (from using a simple jar and water test).

(Note that I am installing the interior weeping tile because I still have some wetness on the interior of the basement wall after an exterior foundation repair last year. This included full mortar repair, waterproofing and membrane down to bottom of wall, and insulation. Due to there being no footing, of course digging could not go below the wall and hence weeping tile level is not below wall - and bottom of wall happens to be floor level. The result is that some wetness is now "wicking" up the bottom of the wall from moist soil underneath. The objective of the interior weeping tile is to generally lower the moisture content at that level, although it is understood that the problem may not go away 100%.)

As you didn't specify, without performing any finite calculations based on existing wall and soil support factors, your engineer was, at best, giving you only an "educated guess" as to the numbers he recommended. Although not commonly done, I have known young, inexperienced engineers who furnished very conservative values in response to someone asking their opinions. Having been a licensed P.E. for more than 40 years, I would never consider doing so without knowing the specifics involved, such as wall dimensions, strength, reinforcement, soil bearing capacity, etc.

I have to ask the obvious question--have you made any serious efforts to minimize the amount of groundwater getting to the perimeter of the house? Such as proper ground slope away from the walls, longer downspout extensions, impervious soil barriers, etc. Can't help but think that you missed a good opportunity to put in an external drain and sump system when the exterior retrofit work was done that you mentioned. Removing water (by pumping it to daylight) that gets to the base of your basement walls before it can seep under the floor might have been a better way to go, and it still could be a cost-effective option, instead of tearing up the basement floors.

I clarified today that apparently the "theoretical rule" is 1:1 slope away from the footing (the wall in my case) according to the engineer. He also indicated that we should go more, and that 1:3 is fine. Yes, there may be some safety factor added here to make it 1:3. In this case, the tentacles (or spurs) every 5 feet would help reach closer to the walls, and he said they would be ok at the 1:1 distance (i.e. 1 foot from wall). The engineer obviously did not do serious calculations. However, given the variable nature and need to add a safety factor, I am ore comfortable with the 1:3.

Regarding the moving of ground water: Yes, we did everything that we could. We sloped the ground away from the house, there are rain gutters with water moving away from the house, and the new driveways on either side of the house slope away from the walls. I believe that it is a combination of ground water (water table rising in springtime) and water from above that is the cause. For example, in 2 cases of strong rain in April and early May, the wicking on the wall all of a sudden went up noticeably.

What exactly do you mean by an external drain system? The repair work last year did include a weeping tile, however it could not be installed low enough next to the wall... so it is now somewhat redundant. Is the external drain you refer to further away from the house and lower than the basement floor? Part of the problem now is that there are driveways on both sides of my house right against the house (old neighbourhood). Maybe this could have been an option when all was dug up, but not cost effective now. Could this be done only on the one side of the house that is most problematic? This is the backside, which is where more water is coming from due to higher ground.
I agree with you that it would have been a better option... but we did not know then what we know now (maybe my contractor should have... but that is now history).

What exactly do you mean by an external drain system? . . . . . Is the external drain you refer to further away from the house and lower than the basement floor? . . . . Could this be done only on the one side of the house that is most problematic? This is the backside, which is where more water is coming from due to higher ground.

I'm not a groundwater expert, nor do I have a strong geological background. But my years of construction experience have consistently shown that accumulating rainwater and melting snow react to the forces of gravity, and tend to follow both the terrain and move towards any natural or man-made voids.

IMHO, an external drain system for your application could consist of grading a natural swale away from the house, approximately parallel to the rear wall of the property while curving around the house. You could construct it with adequate pitch to promote flow, and then terminate either at a daylight outfall or a large sump. For maximum efficiency, you should install a flexible, perforated pipe in a trench at the bottom of the swale (the deeper, the better, to attract more groundwater away from your basement). Placing a layer of washed rock in the bottom of the trench, then a filter-fabric wrap to lay the pipe in to keep the fines from plugging things up (surrounded by more rock), followed by more rock and then topsoil. Depending on your yard's geometry, you might also need a low retaining wall along the far side of the swale. If you have a cheap source of large rock, you could get creative and make a water feature out of the swale (but still have the buried perforated pipe under it).

Over the years, I've built several drainage swales, and they all did an excellent job of keeping groundwater away from the owners' houses (most were crawlspaces, only one a basement, as I recall).